Genetic fingerprinting (A-level only)
AQA
A-Level
Variable number tandem repeats (VNTRs) are loci where a short base sequence is repeated end to end. The repeat sequence is identical between individuals, but the number of times it is copied varies, creating different alleles. Many VNTRs occur in non-coding DNA; because these often do not alter polypeptides, changes in repeat number are usually not selected against, allowing many alleles to accumulate. A person inherits one allele at each locus from each parent. While unrelated people might share a repeat number at a single locus, testing multiple independent loci multiplies the separate probabilities. This makes a match at every locus vanishingly improbable, except in monozygotic twins who share identical DNA.
15
Objectives
10
Exam Tips
15
Pitfalls
20
Key Terms
20
Mark Points
Subtopics in this area
An organism’s genome contains many variable number tandem repeats (VNTRs). The probability of two individuals having the same VNTRs is very low.
The technique of genetic fingerprinting in analysing DNA fragments that have been cloned by PCR, and its use in determining genetic relationships and in determining the genetic variability within a population.
The use of genetic fingerprinting in the fields of forensic science, medical diagnosis, animal and plant breeding.
Students should be able to: explain the biological principles that underpin genetic fingerprinting techniques interpret data showing the results of gel electrophoresis to separate DNA fragments explain why scientists might use genetic fingerprinting in the fields of forensic science, medical diagnosis, animal and plant breeding.
Genetic fingerprinting (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Describe a VNTR as a short base sequence repeated in tandem, stating that individuals differ in the number of repeats.
- Explain why many different repeat numbers exist, linking this to many VNTRs being non-coding and therefore often not selected against.
- Calculate the probability of a chance match across several VNTR loci by multiplying the individual probabilities.
- Explain why genetic fingerprinting cannot distinguish two monozygotic twins.
- Describe the genetic fingerprinting sequence, from PCR amplification through electrophoresis and probe hybridisation to visualisation.
- Explain why shorter DNA fragments travel further through the gel during electrophoresis.
- Use the number of shared bands between two individuals to judge how closely related they are.
- Use the number of different band positions across sampled individuals to determine genetic variability within a population.
- Explain how forensic scientists use VNTR band patterns to reach conclusions about crime scene samples.
- Determine paternity from a set of band patterns by tracing each of the child's bands to a parent.
- Suggest why breeders use genetic fingerprinting to measure genetic diversity and prevent inbreeding.
- Describe a medical application of fingerprinting, such as identifying pathogen strains or diagnosing repeat expansion disorders.
- Explain the biological principles of genetic fingerprinting, including the roles of VNTRs, restriction endonucleases or PCR, gel electrophoresis, Southern blotting and DNA probes.
- Interpret gel electrophoresis data by comparing fragment distances against a lane of known standards.
- Explain why scientists use genetic fingerprinting in forensics, medical diagnosis, and breeding to assess genetic relationships.
Marking Points
Key points examiners look for in your answers
- one mark for describing a VNTR as a short base sequence repeated in tandem at a locus
- one mark for stating that it is the number of repeats, not the repeat sequence, that varies between individuals
- one mark for explaining that many VNTRs are non-coding, so changes in repeat number often do not affect the phenotype and are not selected against
- one mark for explaining that loci are inherited independently, so probabilities at each locus multiply, making a chance match across many loci vanishingly small
- one mark for identifying monozygotic twins as the exception because their DNA is identical
- one mark for using PCR to amplify specific VNTR fragments
- one mark for separating the DNA fragments by length using gel electrophoresis
- one mark for adding labelled DNA probes that bind to the fragments by complementary base pairing/DNA hybridisation
- one mark for identifying the bands by fluorescence or radioactivity
- one mark for stating that more shared bands indicate a closer genetic relationship, or that more varied band positions indicate greater genetic variability in a population
- one mark for a forensic use, comparing VNTR band patterns from a crime scene with a suspect to establish a high probability of presence
- one mark for a paternity use, justifying relatedness by matching each of the child's VNTR bands to either the mother or the alleged father
- one mark for a medical diagnosis use, such as identifying specific strains of pathogens or diagnosing conditions caused by altered tandem repeats
- one mark for a breeding use, comparing band patterns to select the least similar individuals for mating, thereby increasing genetic diversity and preventing inbreeding
- Explain separation: DNA is negatively charged due to phosphate groups and moves towards the anode.
- Explain separation: the gel acts as a sieve, so shorter DNA fragments travel faster and further.
- Explain principles: DNA is cut with restriction endonucleases, or specific VNTR loci are amplified by PCR, producing fragments whose length depends on the number of repeats.
- Explain detection: fragments are transferred to a membrane by Southern blotting and detected using labelled DNA probes.
- Explain uses: in breeding, comparing VNTR patterns identifies genetically diverse individuals to cross, preventing inbreeding.
- Interpret gels: estimate the length of an unknown DNA fragment by comparing its position against a DNA ladder of known standard lengths.
Examiner Tips
Expert advice for maximising your marks
- 💡Put the word 'number' in your answer; the variation is in the number of tandem repeats, and answers that only say the sequence 'varies' rarely score.
- 💡When asked why fingerprinting is reliable, name more than one locus and say the probabilities multiply, rather than just asserting that everyone is different.
- 💡If a question supplies a probability for each locus, multiply across the loci rather than adding them, and write the product out to show your working.
- 💡When describing the method, follow the logical sequence: amplify using PCR, separate using electrophoresis, hybridise with a labelled probe, and visualise.
- 💡State the label and how it is seen, for example a fluorescent probe viewed under UV or a radioactive probe exposed to X-ray film.
- 💡To answer questions on genetic variability, explicitly link a greater number of different band positions in a sample to a larger number of alleles in the population.
- 💡Always state that a forensic match provides a high probability, not absolute certainty, as identical twins share VNTR profiles.
- 💡When explaining breeding uses, specify that comparing VNTR patterns helps identify genetically distant individuals to prevent inbreeding depression.
- 💡When interpreting a gel, explicitly quote the figure's labels (e.g., 'the band at 3 kb in lane 2') rather than making vague comparisons.
- 💡For 'explain why' questions on fingerprinting uses, always provide both the specific field application (e.g., forensics) and the biological reason (e.g., matching VNTR profiles to place a suspect at the scene).
Common Mistakes
Pitfalls to avoid in your exam answers
- describing VNTRs as genes, or saying the repeats code for a protein
- saying the base sequence of the repeat varies between people, when it is the number of copies of that repeat that varies
- writing that every person's VNTR pattern is unique without excluding identical twins
- adding the probabilities from separate loci instead of multiplying them, which makes a chance match look more likely
- forgetting that each VNTR locus carries two alleles, one inherited from each parent
- stating that restriction endonucleases are used to cut the DNA after PCR amplification of VNTRs, which is unnecessary as PCR already produces fragments of the target length
- writing only that the probe 'is complementary' without saying that it binds or hybridises to the DNA
- stating that DNA moves towards the negative electrode, when it is negatively charged and so moves towards the positive electrode
- leaving the DNA double stranded before adding the probe, so there is no exposed base sequence for the probe to pair with
- confusing genetic fingerprinting (analysing VNTRs) with genetic screening (using allele-specific probes to find specific disease alleles)
- stating a fingerprint proves guilt, rather than showing a high probability that the DNA belongs to the suspect
- treating a child's bands as a blend of parents' bands rather than discrete bands inherited whole from each parent
- Stating that PCR alone replaces restriction endonucleases in all genetic fingerprinting. Correction: classical genetic fingerprinting uses restriction endonucleases to cut DNA, while PCR amplification of specific VNTR loci is a modern alternative, so describe the method the question asks about.
- Concluding a definitive match from a single shared band, rather than recognising that a match across multiple VNTR loci is required for a high probability of identification. Correction: state that several loci must match before a match is treated as strong evidence.
- Stating that genetic fingerprinting sequences the whole genome, rather than remembering that it only analyses the length of specific non-coding VNTR fragments. Correction: describe it as comparing fragment lengths at specific VNTR loci, not whole-genome sequencing.